Method of cryosorption pumping



United States Patent Ofiiice 3,332,213 Patented July 25, 1967 3,332,213 METHQD OF CRYOSORPTION PUMPING Frederick Salvatore Di Paolo, Buffalo, and Silviu Alexander Stern, Eggertsville, N.Y., assignors to Union Carbide Corporation, a corporation of New York No Drawing. Filed Aug. 19, 1964, Ser. No. 390,721 6 Claims. (Cl. 55-75) This invention relates to a method for removing gases from a chamber and particularly to a method for evacuating a chamber by cryosorption pumping.

Cryosorption pumping comprises the use of an activated adsorbent which is refrigerated to a selected low temperature in the cryogenic region of 100 K. and below to remove gases from a chamber. Cryosorption pumping to create a vacuum pressure in a chamber differs from mechanical pumping in that certain gases are more strongly sorbed and others less strongly sorbed depending on the adsorbent material used. Further, low temperature refrigeration is a necessity inasmuch as adsorbents have relatively small affinity and capacity for sorbable gases at ambient temperatures compared to their markedly enhanced aflinity and capacity for such gases at low, cryogenic temperatures. As a typical example, adsorbents such as silica gel, charcoal, and activated zeolitic molecular sieves refrigerated to about 7090 K. (liquid nitrogen temperatures) are commonly used to adsorbgases from chambers, molecular sieves being preferred because of their large affinity and capacity for oxygen and nitrogen gases. However, these adsorbents at liquid nitrogen temperatures do not appreciably adsorb helium, neon or hydrogen and hence the absolute pressure to which a chamber can be evacuated by these adsorbents is limited by the partial pressures of these residual gases. With typical liquid nitrogen refrigerated molecular sieve cryosorption pumping techniques, a chamber originally containing atmospheric air could be evacuated no lower than about -10" torr. To achieve lower pressures on the order of 10- torr and lower, common practice has been to flush the chamber with an inert gas such as nitrogen to displace the nonsorbable gases before connecting the liquid nitrogen refrigerated cryosorption pump. Inert gas flushing, however, is time consuming and inconvenient and employed only when dictated by necessity.

The use of liquid nitrogen refrigerated cryosorption pumps has been termed rough pumping to distinguish over the use of cryosorption pumps at liquid hydrogen and helium temperatures to achieve ultra-high vacuums approaching the vacuum of outer space. Both preferably employ molecular sieves, the primary difference being the greater afiinity and capacity at very low temperatures for gases considered nonsorbable at liquid nitrogen temperatures.

It is an object of this invention to provide a cryosorption method which permits rapid attainment of high vacuums. Another object is to provide a cryosorption method which permits attainment of higher ultimate vacuurns than could otherwise be obtained with the same equipment without employing inert gas flushing. A further object is to provide a method for improving the op eration of cryosorption rough'pumps. These and other objects and advantages of this invention will be apparent from the following description.

This invention comprises a method for evacuating a chamber containing a mixture of sorbable and nonsorbable gases by cryosorption pumping in two pumping stages. The first stage is terminated and the second begun When the pressure in the chamber is in the range of two to three orders of magnitude greater than the pressure corresponding to the molecular mean free path equal to the smallest cross-sectional dimension of the conduit providing gaseous communication between the chamber and the first stage cryosorption pumping apparatus. This specific chamber pressure is hereinafter referred to as the critical pressure range.

For any given first stage cryosorption pumping apparatus, this critical pressure range can be easily determined, for example, by calculation from formulae known to those skilled in the art, or by reference to a table plotting the relationship between the molecular mean free path and the corresponding absolute pressure. The January 1963 issue of International Science and Technology on page 47 provides such a graph plotting the mean free path in miles (and in inches where appropriate) and the corresponding pressure in torr, the standard pressure unit employed in vacuum technology.

It has been discovered that the two stage method of this invention permits attaining vacuum pressures several orders of magnitude lower than can be attained by ordinary cryosorption. It has also been discovered that the two stage method of this invention permits the attainment of vacuum pressures below the theoretically obtainable pressure based on the capacity of the adsorbent material for the sorbable and nonsorbable gases at the temperature to which the adsorbent is refrigerated. As an example of these features, a cryosorption pump containing 2.93 pounds of Linde molecular sieve 5A A inch pellets with a moisture content of about 2.2 weight percent, was connected to a chamber containing 1.1 cu. ft. (STP) of atmospheric air by a 1 inch I.D. conduit having an angle valve the-rein. With the angle valve closed, the pump was pre-cooled with liquid nitrogen for ten minutes, and then the angle valve opened. When the desired chamber pressure was reached in the first stage the angle valve was closed, the liquid nitrogen removed, and the pump warmed to room temperature to permit desorbing air to pass through the pump relief valve. With desorption complete, the pump was again pre-cooled for ten minutes and the angle valve opened to begin the second stage. The results of three tests are provided in Table I. I

The critical pressure range of this test system as defined above was between about 0.5 to 5 torr. Thus, in test No. 1 the first stage was terminated at the critical pressure range and after minutes of second stage operation the chamber pressure was 3X10- torr and decreasing at a rapid rate. In test No. 2, the first stage was terminated at a pressure considerably lower than the critical pressure range and the final chamber pressure stabilized at l.5 l torr after 200 minutes. In test No. 3, the first stage was terminated at a still lower pressure and the final chamber pressure stabilized at 6X torr after 100 minutes.

From the foregoing table, an apparent anomaly ap pears to be that the lower the pressure achieved by the first stage, resulting in the adsorption of the largest quantity of gas from the chamber in the first stage, the higher the final chamber presure; and in fact that the lowest pressure achieved by the first stage resulted in the highest final chamber pressure, when it might normally be expected that a lower pressure after termination of the first stage would permit the second stage to achieve the lowest pressure since there would be less gas in the chamber for the second stage to remove. The explanation for the apparent anomaly is believed to be as follows. Until the pressure within the chamber reaches the critical pressure range, the gas molecules are in the viscous regime. Therefore, considering the foregoing example, when the sorbable oxygen and nitrogen gases from the chamber are removed by the cryosorption pump, the nonsorbables-specifically helium, neon and hydrogen are also drawn into the confines of the pump where they are retained. When the chamber pressure falls below the critical pressure range the gas molecules are in the transition regime (e.g. in test No. 2) or the molecular regime (e.g. in test No. 3) and the nonsorbable gases within the confines of the pump migrate back into the chamber. Since the chamber pressure will be lowered below the critical pressure range during the second stage, none of the nonsorbable gases will be retained by the second stage cryosorption pump and, unless previously removed, the partial pressure of such gases will limit the chamber pressure to about l0 torr. In test No. 1, the residual partial pressure of helium and neon in the chamber after termination of the first stage had been lowered by a factor of In test No. 3, virtually no change in the partial pressure of these gases was accomplished by stage one. In test No. 2, there was a slight change inasmuch as not all of these gases were able to migrate back into the chamber since the first stage was terminated in the transition regime where some aspects of viscous flow still prevail.

The capacity of a gas molecule to migrate back into the chamber from the confines of the cryosorption pump has been found to increase markedly as the chamber pressure passes through the transition regime. Further the critical pressure range will rarely exceed 1 torr or fall below 10 torr inasmuch as these limits correspond to gas inlet conduit dimensions of approximately 1-3 inches and 10-50 inches respectively. In practical applications, the gas inlet of a cryosorption pump would rarely exceed these dimensional limits.

The two stage method of this invention will find its greatest utility in rough pumping atmospheric air from chambers. However, this invention will be useful to evacuate any chamber containing gas mixtures with nonsorbable components. As one example, chambers containing artificial atmospheres may require evacuation to test various articles or the like.

In the sizing of cryosorption pumps for use with this invention, sufiicient adsorbent must be used so that the adsorbent capacity is consistent with the ultimate vacuum to be produced. The largest amount of gases will be removed from a chamber above the critical pressure range since the rate of adsorption is greatest in the viscous flow regime. Thus, the cryosorption pump used in the second stage need not have as large a capacity as the cryosorption pump employed in the first stage.

Since cryosorption pumps are normally available in standard sizes, it may be necessary to combine the capacities of several pumps, particularly in the first stage, to accomplish the desired evacuation in a reasonably short period of time. Thus, cryosorption pumps may be manifolded together in series or parallel relationship. If manifolded together in series, the pump in use when the critical pressure range is reached must be shut off when the critical pressure range is attained regardless of whether the adsorption capacity of that pump has been fully utilized. Preceding pumps in the series may be operated in any manner whatsoever and may even be operated such that the adsorption phenomenon comes to equilibrium. If the cryosorption pumps are manifolded together in parallel, each of the pumps must be shut off when the critical pressure range is reached, the critical pressure range being dependent on the size of the gas inlet conduit to each respective pump as defined above. Usually, pumps operated in parallel are connected to the chamber by a single gas inlet and in this case, the critical pressure range of the chamber will be the same for all pumps and will be dependent upon the dimension of the single gas inlet conduit.

The first and second stage cryosorption pumps can be operated in various ways. For example, the adsorbent in both first and second stage pumps can initially be placed in gaseous communication with the chamber and only the first stage cryosorption pump refrigerated until the critical pressure range is reached and the first stage pumping terminated at which time the second stage cryosorption pump is then refrigerated. Since the second stage cryosorption pump is not refrigerated during the first stage pumping the second stage cryosorption pump will remain inactive during the first stage pumping. Alternately, both first and second stage cryosorption pumps could initially be refrigerated with only the adsorbent in the first stage cryosorption pump placed in gaseous communication with the chamber until the critical pressure range is reached and the first stage pumping terminated at which time the adsorbent in the second stage cryosorption pump is placed in gaseous communication with the chamber. Furthermore, in the first example above, the adsorbent in the cryosorption pumps of the two stages can be initially placed in direct gaseous communication so that the second stage cryosorption pump can be evacuated during the first stage pumping.

What is claimed is:

1. A method for evacuating a chamber containing a mixture of sorbable and nonsorbable gases by cryosorption pumping in two stages, the first stage of which comprises providing at least one cryosorption pump in gaseous communication with said chamber, such cryosorption pump having a body of activated adsorbent and a conduit permitting gaseous communication between said chamber and the adsorbent body, and terminating said gaseous communication when the pressure in said chamber is in the range of two to three orders of magnitude greater than the pressure corresponding to the molecular mean free path equal to the smallest cross-sectional dimension of said conduit; and the second stage of which comprises providing at least one cryosorption pump in gaseous communication with said chamber to further evacuate said chamber after gaseous communication to the first stage cryosorption pump is terminated, such second stage cryosorption pump having a body of activated adsorbent and a conduit permitting gaseous communication between said chamber and the adsorbent body.

2. A method according to claim 1 wherein the adsorbent bodies in the first and second stage cryosorption pumps are refrigerated to a temperature between about 70-90 K.

3. A method according to claim 1 wherein the adsorbent bodies of the cryosorption pumps of both stages are initially placed in gaseous communication with said chamber; and wherein the cryosorption pump of said second 5 stage is refrigerated after termination of the first stage pumping.

4. A method according to claim 1 wherein the adsorbent bodies in the first and second stage cryosorption pumps are initially refrigerated; and wherein the adsorbent body of the second stage cryosorption pump is placed in gaseous communication with said chamber after termination of the first stage pumping.

5. A method according to claim 1 wherein the adsorbent bodies in the first and second stage cryosorption pumps comprises molecular sieves.

6. A method according to claim 5 wherein the adsorbent bodies comprise type 5A molecular sieve.

References Cited UNITED STATES PATENTS 1/1964 Jepsen et al 55387 3/1965 Feinleib et a1. 55389 OTHER REFERENCES REUBEN FRIEDMAN, Primary Examiner. I. ADEE, Assistant Examiner. 

1. A METHOD FOR EVACUATING A CHAMBER CONTAINING A MIXTURE OF SORBABLE AND NONSORBABLE GASES BY CRYOSORPTION PUMPING IN TWO STAGES, THE FIRST STAGE OF WHICH COMPRISES PROVIDING AT LEAST ONE CRYOSORPTION PUMP IN GASEOUS COMMUNICATION WITH SAID CHAMBER, SUCH CRYOSORPTION PUMP HAVING A BODY OF ACTIVATED ABSORBENT AND A CONDUIT PERMITTING GASEOUS COMMUNICATION BETWEEN SAID CHAMBER AND THE ADSORBENT BODY, AND TERMINATING SAID GASEOUS COMMUNICATION WHEN THE PRESSURE IN SAID CHAMBER IS IN THE RANGE OF TWO TO THREE ORDERS OF MAGNITUDE GREATER THAN THE PRESSURE CORRESPONDING TO THE MOLECULAR MEANS FREE PATH EQUAL TO THE SMALLEST CROSS-SECTIONAL DIMENSION OF SAID CONDUIT; AND THE SECOND STAGE OF WHICH COMPRISES PROVIDING AT LEAST ONE CRYOSORPTION PUMP IN GASEOUS COMMUNICATION WITH SAID CHAMBER TO FURTHER EVACUATE SAID CHAMBER AFTER GASEOUS COMMUNICATION TO THE FIRST STAGE CRYOSORPTION PUMP IS TERMINATED, SUCH SECOND STAGE CRYOSORPTION PUMP HAVING A BODY OF ACTIVATED ADSORBENT AND A CONDUIT PERMITTING GASEOUS COMMUNICATION BETWEEN SAID CHAMBER AND THE ADSORBENT BODY. 